At 7:59 tonight, with the sun starting to set on the still-packed center court, Bjorn Borg completed an enthralling 6-4, 3-6, 6-3, 3-6, 8-6 victory over Vitas Gerulaitis to move within one opponent - Jimmy Connors - of successfully defending his Wimbledon tennis crown.

Connors, the 1974 champion and No. 1 seed, looked less than a world-beater in ending the unlikely saga of 18-year-old John McEnroe, the youngest men's singles semifinalist in Wimbledon's 100-year history, 6-3, 6-3, 4-6, 6-4.

McEnroe, the recent high school grad from Douglaston, N.Y., put up a tougher fight than anyone expected against a lefthander even cockier than he is.

But their match, which never had any sustained rhythm, was all but forgotten by nightfall, as Borg and Gerulaitis lit up the chilly center court with a display of speed and shot-making that will likely be remembered as the finest match of this year's Wimbledon.

For 3 hours and 5 minutes they scampered around the court, stalking each other like swift welterweights, hitting with the hardest and cleverest shots they could muster.

Blazing passing shots, Razor-edged volleys, Touch, Angles, Windblown Iobs that landed on the lines and kicked chalk. Impossible "gets." probing rallies of the type seldom seen on grass courts. This match had a little of everything - a masterpiece of geometry and, ultimately, drama.

Borg, the stolid Swede with a pulse rate of 35, was down a break at 2-3 in the excruciating final set, a point away from a 2-4 deficit. But he broke back with three blistering groundstrokes, two of which forced lunging volley errors, and a clean forehand passing shot down the line.

"I was a little bit scared when he got that break," Borg said. "It was important to break right back the next game because I think if Vitas held for 4-2 he would have won the match.

"You have to be a little bit lucky to win after five sets like this, because the difference is maybe only one point."

Borg, who held serve after four deuces and two break points in the third game of the final set, looked a little discouraged for the only time when his cross-court backhand was called wide at 30-15 in the next game. He argued briefly, the only such incident in a match in which both players were plugged in and concentrating fiercely, and batted a ball toward the linesman at the end of the game, which Gerulaitis held at 15.

Gerulaitis broke at love in the next game with four great shots: a forehand winner, a lunging backhand down-the-line pass as he darted far out of the court, a dipping return that forced an awkward half-volley error, and a clean backhand cross-court return winner on the sideline.

"I thought I was starting to get to him," Gerulaitis said. "He showed a little emotion on that call. At the change-over he swallowed some dextrose, I thought the guy might even be getting tired."

But in the next game, Borg stuck with Gerulaitis like a bad debt. He smacked a running backhand down-the-line pass for 15-15, a forehand for 30-30, an error-forcing backhand for deuce after Gerulaitis had his game point.

It was this 40-30 point that later haunted Gerulaitis. He stayed back on his second serve, instead of following it to the net as he had done throughout, and though he eventually got in but couldn't handle Borg's passing shot, he felt he should have pressed more initially.

Borg got the advantage with another forehand down-the-line pass that sizzled with topspin, off a long, scrambling rally, and broke with one of the backhand cross courts that he cranks two-fisted. Gerulaitis could only defleet it with a backhand volley.

Borg held for 6-5 after three deuces. Then Gurelaitis held. Borg held at love for 7-6; and Gerulaitis, now serving, got himself in trouble at 15-30 by muffing a forehand volley and drilling a high backhand volley long off a mis-hit forehand return.

Borg got to match point with a perfectly disguised backhand lob that Gerulaitis could only leap for and wave at forlornly as it arched over him for a winner. Then Gerulaitis, practically on his knees, popped a forehand volley wide off a short, treacherously sinking return. His heart sank and his head dropped as the ball landed inches wide.

It was over.

Seldom do two players maintain as high a standard as these two did for so long a time, and the crowd of 15,000 that stayed on a breezy afternoon stood and applauded for several minutes.

"Maybe a couple of years ago I would have been happy just to play a match like that," said Gerulaitis, who beat Jan Kodes, Adriano Panatta, Brian Gottfried and Antonio Zugarelli in succession to win the Italian Open his first big international title, last month.

"But today I really wanted to win and get into the final.I didn't let anything upset me. I had one intention and that was to win the match."

Borg played aggressively and powerfully on his own serve, and though he did not volley as much as Gerulaitis, he didn't miss many when they counted.

They both lobbed, scrambled and half-volleyed so well that numerous points went on, all over the court, long after they logically should have ended.

Gerulaitis' serve has improved enormously, especially the second delivery which he now kicks deep and follows in on fast courts. He is so quick and agile at the net that he can devour or angle away shots that would pass practically anyone else.

Borg, 21, and Gerulaitis, 22, are similar in build, continentally tapered with slender waists and big shoulders. They both have tousled blond manes. Although Borg is an impassive Scandinavian and Gerulaitis a flamboyant New Yorker of Lithuanian descent. They have sometimes been mistaken for one another by squealing autograph-seeking schoolgirls.

Connors played relatively well the first two sets, making McEnroe pay for his inability to get his first serve in on big points.

But in the next two sets, he struggled. His forehand volley was woeful. His backhand volley was shaky as well. He made far too many unforced errors.

"If Jimmy plays the way he did today, and Borg plays the way he did (today) in the (Saturday) final, Borg will kill him," said Gerulaitis.

But now that Wimbledon, after all the upsets, has come down to the No. 1 and No. 2 players in the world in a rematch of the U.S. Open final that decided the No. 1 world ranking last year (Connors won), the challenge can be expected to raise the level of Connors' game.

The biggest question is can Borg produce another effort equal to the one it took to win today?

Meanwhile, the women's title will be decided Friday as Virginia Wade carries the hopes of her British countrymen against Betty Stove of The Netherlands.